The invention concerns a process for detecting combustion irregularities in an internal combustion engine in order to make a combustion diagnosis, particularly at medium and high speed. It extends to a device for implementing said process and to injection engine control systems equipped with such devices.
Combustion irregularities in an internal combustion engine (misfires and partial combustion) are interesting to diagnose because they are indicative of various disorders liable to affect either the preparation of the air/fuel mixture, or the ignition means, or a mechanical component (tightness of a valve). Moreover, these combustion defects increase pollutant emissions and can lead to external combustion causing damage to catalytic exhaust systems.
The following vocabulary is used below:
active TDC (active top dead center): angular position of the crankshaft which precedes the expansion of the gases, corresponding to the minimum distance between the piston and the top of the cylinder head, PA1 combustion phase of a cylinder: phase during which the explosion of the gases occurs after ignition, this phase extending at medium and high speeds from ignition to about half of expansion, PA1 active cylinder: cylinder in which combustion takes place, PA1 cycle: time interval between two active TDC of a cylinder. PA1 (a) for each phase of combustion of the engine, a sampling window is defined, centered on an angular position, called the reference position, corresponding to a predetermined moment of the combustion phase concerned, said sampling window exhibiting a width less than twice the angular distance between two combustions, PA1 (c) for each set of samples, a linear combination is made of the values measured .DELTA.T.sub.1, . . . .DELTA.T.sub.n in order to obtain a parameter, called the critical parameter, which is representative of the variation in the instantaneous speed at the moment of combustion, PA1 (d) the critical parameter resulting from the linear combination is compared with the above-mentioned fault threshold, and information is transmitted representative of a fault if said threshold is exceeded. PA1 (b) in each sampling window, the measurements of time intervals are symmetrically distributed about the reference position, in order to obtain approximately half of the samples .DELTA.T.sub.1, . . . .DELTA.T.sub.n/2 before the reference position and the other half of the samples .DELTA.T.sub.n/2+1, . . . .DELTA.T.sub.n after said position, PA1 (c) a linear combination is made of the samples by assigning negative coefficients to the first half of the samples .DELTA.T.sub.1, . . . .DELTA.T.sub.n/2 and symmetrical positive coefficients to the second half of the samples .DELTA.T.sub.n/2+1, . . . .DELTA.T.sub.n. PA1 (b) in each sampling window, three measurements .DELTA.T.sub.1, .DELTA.T.sub.2 and .DELTA.T.sub.3 are taken before the reference position and three measurements .DELTA.T.sub.4, .DELTA.T.sub.5 and .DELTA.T.sub.6 after said position, PA1 (c) the following linear calculation is carried out to obtain the critical parameter: EQU -.DELTA.T.sub.1 -3.DELTA.T.sub.2 -2.DELTA.T.sub.3 +2.DELTA.T.sub.4 +3.DELTA.T.sub.5 +.DELTA.T.sub.6 PA1 by artificially causing a lack of combustion for predetermined speeds and loads, PA1 by taking measurements of time intervals and by calculating the corresponding physical parameters, PA1 and by memorizing a fraction of these parameters as fault thresholds. PA1 the angular position sensor and the counter are adapted to provide a time resolution less than 1 .mu.s, PA1 the computation means are programmed:
Partial combustion in a given cycle causes a variation of the drive torque in an engine and hence a variation in the engine speed in comparison with the average speed considered over many cycles. Today, it is known how to detect these speed variations, on the one hand, at low engine speed (below 3000 rpm), irrespective of the load applied, and, on the other, at medium speed under high load. For example, the publication by Gunther Plapp et al, `Methods of on-board misfire detection, SAE 900232, 1990`, describes a process in which the average engine speed is compared during successive combustion phases in order to detect significant variations. However, at high speed or under low load, these variations in average speed have relatively low values and are impossible to detect. Furthermore, irrespective of the speed, these relative variations are not necessarily indicative of a combustion irregularity, and may be confused with variations in the engine speed. To make a reliable diagnosis, it is accordingly necessary to equip the engine with an additional displacement transducer (French Patent No. 91.12743) or with an accelerometer, to invalidate the fault detection in the transient phases. Another publication, by W. B. Ribbens and C. Rizzoni, `Applications of precise crankshaft position measurements for engine testing, control and diagnosis, SAE 890885, 1989`, describes a process in which matrix algebra calculations are carried out on a set of instantaneous speed measurements taken during each engine cycle, in order to determine the absolute value of the drive torque for each cylinder and thus to determine abnormal variations in said torque. However, these calculations are highly complex and demand powerful computation resources, which cannot conceivably be mounted onboard an automotive vehicle. Moreover, according to this publication, it appears that the process is only usable at low speed (less than 3500 rpm).